Add changelog update instructions to Copilot guidelines

[Copilot]

This PR adds comprehensive changelog update instructions to the Copilot guidelines to ensure all pull requests include proper changelog updates following the Keep a Changelog format.

Changes Made

Enhanced Copilot Instructions

• Added a new "Changelog Updates" section to the Development Workflow
• Updated the "Making Code Changes" process to include changelog updates as a required step
• Enhanced "Common Validation Steps" with changelog verification checklist

Key Requirements Added

• Mandatory for every PR: Update CHANGELOG.md in the "Unreleased" section
• Standard format: Follow Keep a Changelog format
• Clear categories: Use appropriate sections (Added, Changed, Deprecated, Removed, Fixed, Security)
• User-focused descriptions: Write clear, concise entries that help users understand changes

Documentation Structure

The instructions include:

• Clear process steps for updating the changelog
• Complete list of changelog categories with descriptions
• Example changelog entry showing proper formatting
• Validation checklist to ensure quality

Example Usage

## [Unreleased]

### Added
- New PowerShell localization parsing feature for multi-language support

### Fixed
- Resolved issue with decoration provider not updating on file changes

This ensures consistent changelog maintenance across all contributions while maintaining the existing high-quality documentation standards in the repository.

Fixes #8.

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